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RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ...
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March 1955 Popular ElectronicsTable of Contents
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Popular Electronics was published from October 1954 through April 1985. All copyrights are hereby acknowledged. See all articles from Popular Electronics.articles from Popular Electronics.
As legend goes, the use of microwaves for preparing food was pursued after a serendipitous discovery by Raytheon engineer Percy Spencer whereby he noticed the chocolate bar in his pocket had melted while he was working near a radar transmitter magnetron. Being a newly discovered phenomenon in 1945, Mr. Spencer was probably not aware that his own body parts were being likewise cooked, but he did recognize the commercial potential of an oven that used microwaves to cook food. It only took Raytheon (Amana) to have the first Radarange available for sale to professional kitchens. This article was printed a full decade after the discovery and even then the size and power consumption was too great for grandma's countertop. Of course grandma would never even have considered using such a newfangled device. In fact, many people equated the microwave's cooking action with using a nuclear reaction to do the job, hence the term "nuking" food in the microwave. Keep in mind that it was the era of the first atomic bomb explosions where gruesome films were shown burning the flesh off of test animals on ships at the Bikini Atoll. The crucial difference between microwave radiation and nuclear radiation is that microwaves are non-ionizing, meaning that they do not contain enough or the right wavelength energy to knock electrons out of their orbits (yeah, an outmoded term but it sounds cooler than probability density), whereas decaying nuclear products do. I remember my mother's first microwave oven (c.1975) was huge and weighted probably 30 pounds; it occupied the full depth of a standard countertop (25"). For many years there was a concern about microwave radiation leaks around the door gaskets and even the chassis, but that issue has been resolved in modern appliances. In fact, with microwave ovens operating in the same 2.4 GHz frequency band as Bluetooth and WiFi, there was also a big discussion in the mid-1990s about whether microwave ovens would interfere with and even prevent those technologies from being usable. Just as Y2K (remember that?) turned out to be a non-event, so did microwave oven interference. BTW, I took a picture of one of the original Radaranges on display in the National Electronics Museum at the 2009 MTT-S shown in Boston.
Only the food gets hot in the new "Radarange". Cooking utensils and interior of oven remain cool since these materials do not absorb the microwaves that cook the food.
Food can be cooked in a fraction of the time it usually requires, by the new "Radarange" which utilizes microwave energy to heat nothing but the food itself. Made by the Raytheon Manufacturing Company of Waltham, Mass., the new oven can prepare an 18-pound roast of beef in 40 minutes. At a recent demonstration, a chicken was roasted in 9 minutes; an apple pie baked in 6 minutes; and steaks done in 1 minute. The walls of the oven, as well as the utensils holding the food, remained cold and could be touched with bare hands.
Designed for primary cooking, defrosting, and reheating, the "Radarange" heats food by microwave energy generated at 2450 megacycles, produced by QK-390 continuous wave, air-cooled magnetrons. In model 1161, two magnetrons produce a maximum of 1600 watts; in smaller model 1170, one magnetron produces a maximum of 800 watts. In both cases the microwave energy is directly coupled to the oven cavity, where it is confined by the metal walls and a door designed with appropriate chokes. Thus, instead of the food being cooked by the conventional method of applying heat to its surface and then waiting for the heat to be conducted through the food, the food in the "Radarange" oven is penetrated by the microwaves to a depth of about 2 1/2 inches. As it penetrates, the microwave energy sets up molecular friction deep within the food which in turn creates the heat that cooks it. This process is the key to the tremendous reduction in time needed for cooking the food.
No physical change takes place in the food, except the normal changes caused by the heat. And the only heat present is within the food itself. Since the stainless steel of the oven, and the material of which cooking utensils and plates are made resist microwave penetration, they do not get hot.
The "Radarange" oven's tremendous speed helps reduce food wastage. For example, in estimating a restaurant's daily needs for roasts, only enough meat for the smallest expected amount of business is cooked by conventional methods. Should extra business develop later in the day, extra roasts may be prepared in the "Radarange" oven in ample time to meet the orders. In most establishments, this procedure may be used repeatedly, keeping just "one roast ahead" of business.
Except for the magnetrons, all the electrical equipment in the "Radarange" runs at power frequencies and consists of power supply and control equipment. The only tubes besides the magnetrons are the rectifier tubes in the power supply, which furnishes 320 ma at 5000 volts to each magnetron.
Magnetrons are essentially constant voltage devices like gas voltage regulator tubes and will draw widely fluctuating amounts of current with very small changes in voltage. Therefore, some means of current control is required. This control is accomplished very simply with a saturable reactor circuit in series with the primary of the high-voltage transformer. Using this reactor, the magnetron circuit, and thus the cooking speed, is held virtually constant for changes in line voltage of plus 10 or minus 5 percent from the design voltage. The reactor is also used to provide lower oven heats by reducing the magnetron current. This method is a particularly convenient way to control heats, as all switching can be done in the low current control circuit rather than in the power circuit.
Power is provided for exhausting steam from the oven and cooling magnetrons and other components. Considerable care has been taken to insure long life by running the electrical components cool. The air in commercial kitchens is usually hot, grease laden, and frequently full of lint. The electronic components in the range operate at relatively high power and must be kept cool if long and trouble-free operation is to be achieved. Cooling air, therefore, is drawn in at the front, where the air is most apt to be cool and free of grease when the range is placed in a row of other cooking equipment. The air is then filtered and forced over the electrical components and out through openings in the back of the cabinet.
The microwave ovens are designed to operate on 208 to 230 volts, single phase, 60 cycles. The neutral wire carries no current and can be used for grounding. The conversion from 208 to 230 volts is made by changing a transformer tap within the range. The large model consumes about 1.02 kw on standby and about 5.3 kw on high heat. The power demand for the small model is half of that of the large range.
The efficiency of the magnetron itself is about 50 percent; that is, about half of the power supplied to the magnetron is fed into the oven as microwave power. Over 90 percent of this power is converted to heat in the food. The remaining 10 percent is dissipated in random heating.
Nuclear Bomb Test at the Bikini Atoll
Posted January 28, 2014